Noticed this driving the brand new car home from the dealership just after purchase. Interstate at exactly 75 mph I notice passenger seat shaking/vibrating. Felt a little bit of vibration in driver's seat too between 70-75.

Thought it might be the tire pressure so I decreased them from around 40 (how they came) to 34/33 the following day and tested again on the interstate and vibration/shaking is still there. Will be bringing back to the dealer to check it out. I've already spoken to them.

Hopefully just a tire balance/out of round issue. My car was a Sept 2012 build so it could feasibly have been sitting on the lot for 5 months before I purchased it. Is this enough to cause flat spotting? Not a great feeling after having dealt with the dreaded steering wheel vibration in my '11 Outback.